A predetermined fill level can be monitored, for example, by means of the conductive measuring method. This basic measuring principle is known from a number of publications. Fill level is monitored by detecting whether an electrical contact is established via the conductive medium between the sensor electrode and the wall of a conductive container or between the sensor electrode and a second electrode. Corresponding field devices are sold by the applicant, for example, under the mark, Liquipoint.
Fill level detection by means of a conductive measuring method reaches its limits, when the medium to be monitored has virtually no electrical conductivity (<0.5 μS/cm) or only a very low conductivity. A change of the conductivity of the medium relative to the conductivity of air is then too small to be able to be registered reliably by the measuring electronics. Media difficulty monitored with a conductive measuring method include e.g. distilled water, molasses and alcohols. Further problematic are media with an electrical conductivity of less than 1 μS/cm and a dielectric constant of less than 20. Falling in this range fall are especially oils and fats.
Suited in such case is the capacitive measuring method, which is likewise known from the state of the art. In such case, the fill level of the medium is ascertained from the capacitance of the capacitor formed by a probe electrode and the wall of the container or a second electrode. Depending on conductivity of the medium, either the medium or a probe insulation forms the dielectric of the capacitor. Also field devices based on the capacitive measuring principle are sold by the applicant in many different embodiments, for example, under the marks, Liquicap and Solicap.
Fill level detection by means of a capacitive measuring method is, indeed, possible, in principle, for both conductive and non-conductive media. However, for media with an electrical conductivity >50 μS/cm, insulation of the measuring probe is necessary. The impedance of this insulation is, in turn, disadvantageous in the case of clinging or accreting media.
Known from German Patent, DE 32 12 434 C2 for preventing accretion formation is the application of a guard electrode, which coaxially surrounds the sensor electrode and lies at the same electrical potential as the sensor electrode. Depending on the character of the accretion, there is in the case of this embodiment the problem of suitably producing the guard signal.
Furthermore, described in German Patent, DE 10 2006 047 780 A1 is a fill level measuring probe, which is insensitive to accretion formation over a large measuring range. In this known solution, an amplifying unit and a limiting element are provided, wherein the limiting element is arranged between the output of the amplifying unit and the guard electrode. The guard electrode is supplied via the amplifying unit and the limiting element, which is e.g. a resistor, with a guard signal. The sensor electrode is supplied analogously with the operating signal. An evaluation unit monitors fill level based on the electrical current signal tappable at the sensor electrode and the operating signal and/or the guard signal. The amplifying unit, which produces the guard signal, is limited by the limiting element. The signal, limited in its amplitude, is sent as exciter signal to the sensor electrode. Then, tapped from the sensor electrode is an electrical current signal, which in combination with the operating signal or the guard signal is taken into consideration for the purpose of monitoring the fill level.
Finally, known from German Patent, DE 10 2008 043 412 A1 is a fill level limit switch having a memory unit, wherein stored in the memory unit are limit values for different media located in a container. Upon exceeding or subceeding the limit value for the medium, a switching signal is produced. Especially, the limit value for the measured value can be so established with reference to the medium located in the container that an accretion formation does not interfere with reliable switching. Since accretion formation corrupts the measurement signal and, thus, falsely indicates an incorrect process variable, the limit value (which determines the switching point) is preferably so set that it lies outside of the region attainable for the measurement signal in the case of accretion. The apparatus can be embodied, in such case, as a capacitive or as a conductive, fill-level measuring device. Since the apparatus can automatically adjust to alternating media (e.g. also in the context of cleaning procedures such as CIP- and SIP processes) in the container by ascertaining, respectively calculating, the optimal switching point from the registered properties of the medium, complex adjustment procedures, which are usually necessary in the case of an alternation of the medium, can be omitted.
It would be desirable, if the fill level of a medium in a container could be monitorable with a measuring device independently of the electrical properties of the medium. Since the advantages and disadvantages of the capacitive and conductive measuring methods are opposite, a multisensor is promising, which can monitor fill level by means of both methods. Such a multisensor is distinguished by features including that it permits working alternately in a capacitive and in a conductive operating mode. In such case, a guard electrode can be supplementally provided for preventing accretion formation.
Different options are conceivable for concrete construction of such a field device. For example, a measuring probe with two electronic units can be provided, one for the capacitive and one for the conductive, operating mode. In order to be able to switch back and forth between the two modes, electrical switches can be supplementally installed, for example. This simply implementable example has, however, the disadvantage that the switches limit the achievable measuring resolution due to parasitic capacitance, which can be especially disadvantageous in the case of the capacitive operating mode.
The achievable measuring resolution in the capacitive operating mode depends on the particular geometric embodiment of the measuring probe as well as on the components used for the respective electronics unit. Of course, the measured capacitances depend, moreover, on, among other things, the properties of the medium. However, this dependence concerns the respectively current application, while the geometry of the measuring probe as well as the components of the electronics unit represent a constant influence.
The most important feature is the geometric embodiment of the measuring probe, since this fixes the range of the measured capacitances.
When the measuring probe is, for example, so embodied that after installation in the wall of the container it is flush with the wall, such as in the case of the variant sold by the applicant under the designation FTW33, the measured capacitances can lie in the range of femtofarads. If the measuring probe, in contrast, protrudes at least partially into the container, then the measured values for the capacitance lie up to a number of orders of magnitude thereabove.
Especially an evaluation of capacitances in the femtofarad range places highest requirements on the applied electronics unit.